1. Field of Invention
The invention relates to a recording medium conveying device that conveys a recording medium by nipping the recording medium, and an image forming apparatus including the recording medium conveying device.
2. Description of Related Art
In a conventional image forming apparatus, that forms an image onto a recording medium by ejecting ink droplets from nozzles of an ink jet recording head, the recording medium is intermittently (step by step) moved in a sub-scanning direction by a predetermined distance. While the recording medium is not intermittently moved, a carriage having the recording head is moved in a main scanning direction to form an image in a predetermined area.
In the image forming apparatus, a pair of upstream conveyor rollers and a pair of downstream conveyor rollers are provided upstream and downstream of the recording head in a recording medium conveying direction, respectively. The recording medium is nipped by the pairs of upstream and downstream conveyor rollers and is moved in the sub-scanning direction by intermittently driving both the pairs of conveyor rollers.
Especially in a case where a recording medium is thick, a load that is imposed on the recording medium from a nipping point of the upstream conveyor rollers suddenly becomes zero when a trailing edge of the recording medium is released from the upstream conveyor rollers. With the change of the load, the trailing edge of the recording medium is suddenly pushed by the upstream conveyor rollers, resulting in the recording medium being excessively conveyed against the nipping force of the downstream conveyor rollers. Because of this situation, an image dropout (an image is severed into several pieces) occurs in the recording image in a sheet conveying direction, thereby degrading image quality.
Japanese Laid-Open Patent Publication No. 3-90378 discloses a medium processing device wherein a printing unit is provided between a pair of upstream conveyor rollers and a pair of downstream conveyor rollers, a first detector is provided near the upstream conveyor rollers, and a second detector is provided near the downstream conveyor rollers. Each of a driven roller of the upstream and downstream conveyor rollers includes an engaging and disengaging device (solenoid) to selectively engage and disengage the driven rollers with and from respective drive rollers.
Based on the detection of a leading edge of the recording medium by the first or second detectors, the leading edge of the recording medium is positioned between the drive roller and driven rollers, which are separated from one another. Then, the driven roller is brought into contact with the drive roller so as to be pressed against the drive roller, and the recording medium is conveyed in a downstream direction. Based on the detection of the leading edge and the trailing edge of the recording medium by the first detector, a length of the recording medium is obtained. After that, the leading edge of the recording medium is positioned in a printing position by rotating the -drive rollers in a reverse direction to convey the recording medium in the reverse direction. Then, the upstream conveyor rollers are rotated in a normal direction to perform printing on the recording medium. As the second detector detects the leading edge of the recording medium, the solenoid activates so that the recording medium is nipped by the drive roller and the driven roller of the downstream conveyor rollers and another solenoid activates to disengage the driven roller from the drive roller of the upstream conveyor rollers. After that, the recording medium processing device continues the printing operation in a printing area while the recording medium is nipped by the downstream conveyor rollers.